1. Field of the Invention
This invention relates to a method for pouring molten metal which adjusts the extent of opening of an opening adjusting device, such as a sliding nozzle or the like, provided at a molten metal pouring apparatus so that molten metal stored once in the pouring apparatus is poured into other vessels or molds, and more particularly to a method for pouring molten metal which prevents solidifying or adhesion of molten metal to the opening adjusting device, thereby enabling improvement in control accuracy and prevention of clogging to the nozzle.
2. Description of the Prior Art
When molten metal in a tundish at a continuous casting machine is poured into a mold, the level of molten metal poured therein is measured by use of the radiant ray, ultrasonic wave, thermo couple or TV camera and the extent of opening of a sliding nozzle is automatically adjusted on the basis of the measured values so that the level of molten metal is positioned within the reference allowance range providing a dead zone, thereby carrying out the molten metal level control. Such continuous casting method is well known.
The molten metal level control, as shown in FIG. 1, is carried out in such a manner that a level measuring apparatus 5 is mounted at the rear side of a mold 4 to detect the level of molten metal therein, so that in a case where the detected value measured by the level measuring apparatus 5 is higher than the reference allowance range set by a level setting instrument 8, a control signal generated by an adjuster 9 is given to a servo-amplifier 11 to actuate a servo valve 13, thereby adjusting a sliding nozzle 1 in the closing direction through a servo cylinder 12, a pilot cylinder 12' and a work cylinder 7 so as to reduce a sectional area of a molten metal passage, thus reducing a flow rate of molten metal 2 passing through the sliding nozzle 1 from a tundish 3. On the contrary, in a case where the level of molten metal is lower than the reference allowance range, the sliding nozzle 1 is adjusted in the opening direction similarly through the servo cylinder 12, pilot cylinder 12' and power cylinder 7 so as to enlarge the sectional area of the molten metal passage and increase a flow rate of molten metal 2 passing through the sliding nozzle 1 from the tundish 3, thereby adjusting the level of molten metal to be positioned always within the reference allowance range.
However, when the time of pouring under such level control is long, for example, about 30 minutes after a start of pouring, raw metal, as shown by crosshatching in FIG. 2, getting into a gap between fixed plate 1b and a sliding plate 1a and deposited on the shoulder of inner wall of a sliding nozzle 1 to the sliding plate 1a hinders the sliding plate 1a from slidable motion. Also, the sliding plate 1a is overheated by high temperature of molten metal 2 and distorted, thereby increasing sliding friction at the surface of sliding plate 1a and deteriorating the response to the level control, resulting in that the molten metal level is liable to come out from the reference allowance range.
In detail, a difference (to be hereinafter called the deflection of level) between the levels obtained by the adjuster 9 with respect to the reference allowance range is to be eliminated by the command signal of extent (FIG. 3-(a)) output from the servo-amplifier 11, the servo cylinder 12 operates in response to the command signal as shown in FIG. 3-(b), and the work cylinder 7 is affected by sliding resistance increased by raw metal getting into a gap between the fixed plate 1b and the sliding plate 1a and deposited and growing up therebetween, and by thermal distortion from overheating, thereby not faithfully following the command signal as shown in FIG. 3-(c). Hence, the control accuracy for the level of molten metal lowers as shown in FIG. 3-(d), so that there is a defect in that the level of molten metal may come out from the reference allowance range.